The transplantation of tissue from one mammal to another has been used for years to save lives and to improve the quality of lives. For example, the first successful kidney transplant was performed in the mid-1950s between identical twin brothers. Since then, donors have grown to include not only close relatives but also distant relatives, friends, and total strangers. In some cases, the recipient may reject the transplanted tissue. Thus, tissue rejection is a concern for any recipient of transplanted tissue. If a doctor is able to recognize early signs of tissue rejection, anti-rejection medication often can be used to reverse tissue rejection.